Hybrid Control Apparatus and Method

ABSTRACT

An apparatus comprises a power converter connected between an input power source and a wireless power transfer system, wherein the power converter is configured to regulate a voltage applied to the wireless power transfer system when a load current of the wireless power transfer system is less than a first threshold and the power converter is configured to operate in an always-on mode when the load current of the wireless power transfer system is greater than the first threshold.

TECHNICAL FIELD

The present invention relates to a hybrid control method, and, inparticular embodiments, to a hybrid control method applied to a wirelesspower transfer system.

BACKGROUND

As technologies further advance, wireless power transfer has emerged asan efficient and convenient mechanism for powering or charging batterybased mobile devices such as mobile phones, tablet PCs, digital cameras,MP3 players and/or the like. A wireless power transfer system typicallycomprises a primary side transmitter and a secondary side receiver. Theprimary side transmitter is magnetically coupled to the secondary sidereceiver through a magnetic coupling. The magnetic coupling may beimplemented as a loosely coupled transformer having a primary side coilformed in the primary side transmitter and a secondary side coil formedin the secondary side receiver.

The primary side transmitter may comprise a power conversion unit suchas a primary side of a power converter. The power conversion unit iscoupled to a power source and is capable of converting electrical powerto wireless power signals. The secondary side receiver is able toreceive the wireless power signals through the loosely coupledtransformer and convert the received wireless power signals toelectrical power suitable for a load.

As power consumption has become more important, there may be a need forhigh power density and high efficiency wireless power transfer systems.Resonant converter based wireless power transfer systems have become thepreferred choice for achieving high performance (e.g., lower powerlosses) because resonant converters are capable of reducing switchinglosses of power switches through zero voltage switching and/or zerocurrent switching. However, as the frequency of the wireless powertransfer system goes higher, achieving a high efficiency power wirelesspower transfer system under different loading conditions become asignificant issue, which presents challenges to the system design of thewireless power transfer system.

It would be desirable to have a wireless power transfer control methodexhibiting good behaviors such as high efficiency and lowelectromagnetic interference (EMI) under a variety of loadingconditions.

SUMMARY

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present disclosure which provide a wireless power transfer systemoperating in different control modes in response to different loadcurrents or different loading conditions.

In accordance with an embodiment, an apparatus comprises a powerconverter connected between an input power source and a wireless powertransfer system, wherein the power converter is configured to regulate avoltage applied to the wireless power transfer system when a loadcurrent of the wireless power transfer system is less than a firstthreshold and the power converter is configured to operate in analways-on mode when the load current of the wireless power transfersystem is greater than the first threshold.

In accordance with another embodiment, a method comprises detecting aload current of a wireless power transfer system comprising a powerconverter and a wireless power transfer device connected in cascadebetween an input power source and a load, configuring the powerconverter to regulate a voltage applied to the wireless power transferdevice when the load current is less than a first current threshold andconfiguring the power converter to operate in an always-on mode when theload current is greater than the first current threshold.

In accordance with yet another embodiment, a system comprises aconverter comprising a high-side switch and a low-side switch connectedin series between an input power source and ground, an inductorconnected to a common node of the high-side switch and the low-sideswitch and an output capacitor connected to the inductor.

The system further comprises a wireless power transfer device comprisinga full-bridge connected between an output of the converter and ground, atransmitter coil coupled to the full-bridge, a resonant capacitorconnected between the full-bridge and the transmitter coil, a receivercoil magnetically coupled to the transmitter coil and a rectifierconnected to the receiver coil, wherein the converter is configured tooperate in two different operation modes in response to different loadcurrents of the wireless power transfer device.

An advantage of an embodiment of the present disclosure is a hybridcontrol method applied to a wireless power transfer system.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter which form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a wireless power transfer systemin accordance with various embodiments of the present disclosure;

FIG. 2 illustrates a schematic diagram of the wireless power transfersystem shown in FIG. 1 in accordance with various embodiments of thepresent disclosure;

FIG. 3 is an operating mode control chart illustrating the operatingprinciple of the wireless power transfer system shown in FIG. 1 inaccordance with various embodiments of the present disclosure;

FIG. 4 illustrates gate drive signals associated with a firstimplementation of the minimum duty cycle control mode in accordance withvarious embodiments of the present disclosure;

FIG. 5 illustrates gate drive signals associated with a secondimplementation of the minimum duty cycle control mode in accordance withvarious embodiments of the present disclosure;

FIG. 6 illustrates gate drive signals associated with the minimumphase-shifted control mode in accordance with various embodiments of thepresent disclosure;

FIG. 7 illustrates gate drive signals associated with a firstimplementation of the duty cycle control mode in accordance with variousembodiments of the present disclosure;

FIG. 8 illustrates gate drive signals associated with a secondimplementation of the duty cycle control mode in accordance with variousembodiments of the present disclosure;

FIG. 9 illustrates gate drive signals associated with the phase-shiftedcontrol mode in accordance with various embodiments of the presentdisclosure;

FIG. 10 illustrates gate drive signals having a leading edge alignmentunder the first implementation of the minimum duty cycle control mode inaccordance with various embodiments of the present disclosure;

FIG. 11 illustrates gate drive signals having a first leading edgealignment under the second implementation of the minimum duty cyclecontrol mode in accordance with various embodiments of the presentdisclosure;

FIG. 12 illustrates gate drive signals having a second leading edgealignment under the second implementation of the minimum duty cyclecontrol mode in accordance with various embodiments of the presentdisclosure;

FIG. 13 illustrates gate drive signals having a first leading edgealignment under the minimum phase-shifted control mode in accordancewith various embodiments of the present disclosure;

FIG. 14 illustrates gate drive signals having a second leading edgealignment under the minimum phase-shifted control mode in accordancewith various embodiments of the present disclosure; and

FIG. 15 illustrates a flow chart of controlling the switches shown inFIG. 2 in accordance with various embodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent disclosure provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to preferredembodiments in a specific context, namely a wireless power transfersystem operating in different control modes for increasing transferefficiency and reducing system costs. The invention may also be applied,however, to a variety of power systems. Hereinafter, various embodimentswill be explained in detail with reference to the accompanying drawings.

FIG. 1 illustrates a block diagram of a wireless power transfer systemin accordance with various embodiments of the present disclosure. Thewireless power transfer system 100 comprises a power converter 104 and awireless power transfer device 101 connected in cascade between an inputpower source 102 and a load 114. The wireless power transfer device 101includes a power transmitter 110 and a power receiver 120. As shown inFIG. 1, the power transmitter 110 comprises a transmitter circuit 107and a transmitter coil L1 connected in cascade. The input of thetransmitter circuit 107 is coupled to an output of the power converter104. The power receiver 120 comprises a receiver coil L2 and a rectifier112 connected in cascade. The output of the rectifier 112 is coupled tothe load 114.

The power transmitter 110 is magnetically coupled to the power receiver120 through a magnetic field when the power receiver 120 is placed nearthe power transmitter 110. A loosely coupled transformer 115 is formedby the transmitter coil L1, which is part of the power transmitter 110,and the receiver coil L2, which is part of the power receiver 120. As aresult, power may be transferred from the power transmitter 110 to thepower receiver 120.

In some embodiments, the power transmitter 110 may be inside a chargingpad. The transmitter coil is placed underneath the top surface of thecharging pad. The power receiver 120 may be embedded in a mobile phone.When the mobile phone is place near the charging pad, a magneticcoupling may be established between the transmitter coil and thereceiver coil. In other words, the transmitter coil and the receivercoil may form a loosely coupled transformer through which a powertransfer occurs between the power transmitter 110 and the power receiver120. The strength of coupling between the transmitter coil L1 and thereceiver coil L2 is quantified by the coupling coefficient k. In someembodiments, k is in a range from about 0.05 to about 0.9.

In some embodiments, after the magnetic coupling has been establishedbetween the transmitter coil L1 and the receiver coil L2, the powertransmitter 110 and the power receiver 120 may form a power systemthrough which power is wirelessly transferred from the input powersource 102 to the load 114.

The input power source 102 may be a power adapter converting a utilityline voltage to a direct-current (dc) voltage. Alternatively, the inputpower source 102 may be a renewable power source such as a solar panelarray. Furthermore, the input power source 102 may be an energy storagedevice such as rechargeable batteries, fuel cells and/or the like.

The load 114 represents the power consumed by the mobile device (e.g., amobile phone) coupled to the power receiver 120. Alternatively, the load114 may refer to a rechargeable battery and/or batteries connected inseries/parallel, and coupled to the output of the power receiver 120.

The transmitter circuit 107 may comprise primary side switches of afull-bridge converter according to some embodiments. Alternatively, thetransmitter circuit 107 may comprise the primary side switches of otherconverters such as a half-bridge converter, a push-pull converter andthe like. The detailed configuration of the transmitter circuit 107 willbe described below with respect to FIG. 2.

It should be noted that the converters described above are merelyexamples. One having ordinary skill in the art will recognize othersuitable power converters such as class E topology based powerconverters (e.g., a class E amplifier), may alternatively be used.

The transmitter circuit 107 may further comprise a resonant capacitor.The resonant capacitor and the magnetic inductance of the transmittercoil may form a resonant tank. Depending on design needs and differentapplications, the resonant tank may further include a resonant inductor.In some embodiments, the resonant inductor may be implemented as anexternal inductor. In alternative embodiments, the resonant inductor maybe implemented as a connection wire.

The power receiver 120 comprises the receiver coil L2 magneticallycoupled to the transmitter coil L1 after the power receiver 120 isplaced near the power transmitter 110. As a result, power may betransferred to the receiver coil and further delivered to the load 114through the rectifier 112. The power receiver 120 may comprise asecondary resonant capacitor.

The rectifier 112 converts an alternating polarity waveform receivedfrom the output of the receiver coil L2 to a single polarity waveform.In some embodiments, the rectifier 112 comprises a full-wave diodebridge and an output capacitor. In alternative embodiments, thefull-wave diode bridge may be replaced by a full-wave bridge formed byswitching elements such as n-type metal oxide semiconductor (NMOS)transistors.

Furthermore, the rectifier 112 may be formed by other types ofcontrollable devices such as metal oxide semiconductor field effecttransistor (MOSFET) devices, bipolar junction transistor (BJT) devices,super junction transistor (SJT) devices, insulated gate bipolartransistor (IGBT) devices, gallium nitride (GaN) based power devicesand/or the like. The detailed operation and structure of the rectifier112 are well known in the art, and hence are not discussed herein.

The power converter 104 is coupled between the input power source 102and the input of the wireless power transfer device 101. Dependingdesign needs and different applications, the power converter 104 maycomprise many different configurations. In some embodiments, the powerconverter 104 may be a non-isolated power converter such as a buckconverter. In some embodiments, the power converter 104 may beimplemented as a linear regulator. In some embodiments, the powerconverter 104 may be an isolated power converter such as a forwardconverter. The detailed configuration of the power converter 104 will bedescribed below with respect to FIG. 2.

The implementation of the power converter 104 described above is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications.

FIG. 2 illustrates a schematic diagram of the wireless power transfersystem shown in FIG. 1 in accordance with various embodiments of thepresent disclosure. The wireless power transfer system 100 comprises thepower converter 104 and the wireless power transfer device 101 connectedin cascade between the input power source VIN and the load RL.

The power converter 104 is a step-down power converter (also known as abuck converter). The power converter 104 includes a first switch Q1, asecond switch Q2, an inductor Lo and an output capacitor Co. As shown inFIG. 2, the first switch Q1 and the second switch Q2 are connected inseries between an input power source VIN and ground. The inductor Lo isconnected between the common node of the first switch Q1 and the secondswitch Q2, and the output capacitor Co. Throughout the description, thefirst switch Q1 is alternatively referred to as a high-side switch ofthe power converter 104. The second switch Q2 is alternatively referredto as a low-side switch of the power converter 104.

In some embodiments, both the first switch Q1 is implemented and thesecond switch Q2 are implemented as an n-type transistors as shown inFIG. 2. The gate of the first switch Q1 and the gate of the secondswitch Q2 are configured to receive gate drive signals generated by acontroller (not shown).

It should be noted that the power converter 104 shown in FIG. 2 ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, the firstswitch Q1 may be implemented as a p-type transistor.

The wireless power transfer device 101 comprises a full-bridge 106, aresonant capacitor Cp, a loosely coupled transformer 115 and a rectifier112 connected in cascade between an output of the power converter 104and the load RL. The loosely coupled transformer 115 is formed by thetransmitter coil L1 and the receiver coil L2.

The full-bridge 106 includes four switching elements, namely S1, S2, S3and S4. As shown in FIG. 2, the switching elements S1 and S3 areconnected in series between the output terminal of the power converter104 and ground. Likewise, the switching elements S2 and S4 are connectedin series between the output terminal of the power converter 104 andground. The common node of the switching elements S1 and S3 is coupledto a first input terminal of the transmitter coil L1. The common node ofthe switching elements S2 and S4 is coupled to a second input terminalof the transmitter coil L1 through the resonant capacitor Cp.

According to some embodiments, the switching elements S1, S2, S3 and S4are implemented as MOSFET or MOSFETs connected in parallel, anycombinations thereof and/or the like. According to alternativeembodiments, the switching elements (e.g., switch S1) may be aninsulated gate bipolar transistor (IGBT) device. Alternatively, theprimary switches can be any controllable switches such as integratedgate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO)devices, silicon controlled rectifier (SCR) devices, junction gatefield-effect transistor (JFET) devices, MOS controlled thyristor (MCT)devices, gallium nitride (GaN) based power devices and/or the like.

It should be noted that while the example throughout the description isbased upon a full-bridge converter (e.g., full-bridge converter 106shown in FIG. 2), the implementation of the transmitter circuit 107shown in FIG. 1 may have many variations, alternatives, andmodifications. For example, half-bridge converters, push-pullconverters, class E based power converters (e.g., a class E amplifier)may be alternatively employed. Furthermore, aninductor-inductor-capacitor (LLC) resonant converter may be formed whenthe transmitter coil L1 is tightly coupled with the receiver coil L2 insome applications.

In sum, the full-bridge 106 illustrated herein is limited solely for thepurpose of clearly illustrating the inventive aspects of the variousembodiments. The present invention is not limited to any particularpower topology.

It should further be noted that while FIG. 2 illustrates four switchesS1-S4, various embodiments of the present disclosure may include othervariations, modifications and alternatives. For example, a separatecapacitor may be connected in parallel with each switch of thefull-bridge 106. Such a separate capacitor helps to better control thetiming of the resonant process of the full-bridge 106.

The outputs of the receiver coil L2 are coupled to the load RL through aresonant capacitor Cs and the rectifier 112, which is formed by diodesD1, D2, D3 and D4. As shown in FIG. 2, diodes D1, D2, D3 and D4 form afull-wave diode rectifier coupled between the receiver coil L2 and theload RL. The capacitor Co is employed to attenuate noise and provide asteady output voltage.

It should be noted the rectifier structure shown in FIG. 2 is merely anexample. One person skilled in the art will recognize many alternatives,variations and modification. For example, the diodes D1, D2, D3 and D4may be replaced by four switches.

The wireless power transfer system 100 comprises the power converter 104and the wireless power transfer device 101 connected in cascade. Thepower converter 104 is employed to regulate the voltage applied to thewireless power transfer device 101 when the wireless power transfersystem 100 operates in light load conditions. More particularly, thepower converter 104 is configured to regulate the voltage applied to thewireless power transfer device 101 when the load current of the wirelesspower transfer system 100 is less than a predetermined threshold. Afterthe load current of the wireless power transfer system 100 is greaterthan the predetermined threshold, the power converter 104 is configuredto operate in an always-on mode. During the always-on mode, thehigh-side switch Q1 is always on and the low-side switch Q2 is alwaysoff. Throughout the description, the always-on mode may be alternativelyreferred to as a bypass mode. In some embodiments, the predeterminedthreshold is in a range from about 5% of the full load to about 10% ofthe full load of the wireless power transfer system 100.

During the light load operation, in some embodiments, the full-bridge106 is configured to operate in a minimum duty cycle control mode whenthe load current of the wireless power transfer system 100 is less thanthe predetermined threshold. In alternative embodiments, the full-bridge106 is configured to operate in a minimum phase-shifted control modewhen the load current of the wireless power transfer system is less thanthe predetermined threshold.

After the load current of the wireless power transfer system 100 isgreater than the predetermined threshold, the full-bridge 106 isconfigured to operate in a duty cycle control mode. Alternatively, thefull-bridge 106 is configured to operate in a phase-shifted controlmode. The detailed operating principles of the power converter 104 andthe full-bridge 106 will be described below with respect to FIGS. 3-14.

One advantageous feature of the power converter 104 shown in FIG. 2 isthat the power converter 104 is capable of reducing voltage stresses onthe full-bridge 106 through adjusting the output voltage of the powerconverter 105 during light load operation conditions. As a result, it isnot necessary to have a dedicated EMI filter to satisfy the EMCregulations. Furthermore, after the wireless power transfer system 100enters into a heavy load operating mode, the high side switch Q1 of thepower converter 104 is always on. Such an always-on mode helps to reduceconduction losses of the wireless power transfer system 100 so as toachieve high efficiency.

FIG. 3 is an operating mode control chart illustrating the operatingprinciple of the wireless power transfer system shown in FIG. 1 inaccordance with various embodiments of the present disclosure. Thehorizontal axis of FIG. 3 represents the load current of the wirelesspower transfer system. There may be two vertical axes. The firstvertical axis Y1 represents the output voltage of the power converter104. The second vertical axis Y2 represents the duty/phase of thefull-bridge 106 of the wireless power transfer system 100.

As shown in FIG. 3, the operating mode control chart includes threeportions, namely a first portion 302, a second portion 304 and a thirdportion 306. In the first portion 302, the load current of the wirelesspower transfer system 100 is less than a first predetermined threshold.The power converter 104 is configured to regulate the voltage applied tothe full-bridge 106 in a linear manner as shown in FIG. 3. At the sametime, the full-bridge 106 is configured to operate in a minimum dutycycle control mode or a minimum phase-shifted control mode.

In the second portion 304, the load current of the wireless powertransfer system 100 is less than a second predetermined threshold andgreater than the first predetermined threshold. The power converter 104is configured to operate in the always-on mode. The full-bridge 106 isconfigured to operate in a duty cycle control mode or a phase-shiftedcontrol mode.

In the third portion 306, the load current of the wireless powertransfer system 100 is greater than the second predetermined threshold.The power converter 104 is configured to operate in the always-on mode.The full-bridge 106 is configured to operate in a maximum duty cyclecontrol mode or a maximum phase-shifted control mode (e.g., theduty/phase at 12 shown in FIG. 3).

It should be noted that there are at least three operating modes shownin FIG. 3. The wireless power transfer system 100 is capable of having asmooth transition between different modes depending on the load of thewireless power transfer system. For example, the wireless power transfersystem 100 starts with a light load condition (e.g., the first portion302 shown in FIG. 3). During the light load condition, the powerconverter 104 regulates the voltage applied to the full-bridge 106. Asthe load of the wireless power transfer system 100 increases, the outputof the power converter 104 reaches its maximum voltage (e.g., thevoltage at I1 shown in FIG. 3). The power converter 104 stops switchingand enters into the always-on mode. The full-bridge 106 of the wirelesspower transfer system 100 automatically enters into a duty cycle controlmode or a phase-shifted control mode (e.g., the second portion 304 shownin FIG. 3). Furthermore, as the load of the wireless power transfersystem 100 decreases and the minimum duty or the minimum phase shift isreached, the power converter 104 automatically switches from thealways-on mode to the voltage regulation mode as shown in FIG. 3. Theminimum duty may be alternatively referred to as a clamped duty.Likewise, the minimum phase may be alternatively referred to as aclamped phase.

As shown in FIG. 3, depending on different load currents, thefull-bridge 106 may operate in a minimum duty/phase-shifted control modeor a duty/phase-shifted control mode. The gate drive signals of thefull-bridge 106 operating in the minimum duty control mode will beillustrated in FIGS. 4-5. The gate drive signals of the full-bridge 106operating in the minimum phase-shifted control mode will be illustratedin FIG. 6. The gate drive signals of the full-bridge 106 operating inthe duty control mode will be illustrated in FIGS. 7-8. The gate drivesignals of the full-bridge 106 operating in the phase-shifted controlmode will be illustrated in FIG. 9.

FIG. 4 illustrates gate drive signals associated with a firstimplementation of the minimum duty cycle control mode in accordance withvarious embodiments of the present disclosure. The horizontal axis ofFIG. 4 represents intervals of time. There may be four vertical axes.The first vertical axis Y1 represents the gate drive signal of theswitching element S1. The second vertical axis Y2 represents the gatedrive signal of the switching element S2. The third vertical axis Y3represents the gate drive signal of the switching element S3. The fourthvertical axis Y4 represents the gate drive signal of the switchingelement S4.

Referring back to FIG. 2, the full-bridge comprises four switchingelements S1-S4. When the load current of the wireless power transfersystem 100 is less than a predetermined threshold, the power converteris configured to regulate the voltage applied to the input of thefull-bridge 106. The full-bridge 106 is configured to operate in aminimum duty cycle control mode.

During the minimum duty cycle control mode, the gate drive signal of theswitching element S3 is complementary to the gate drive signal of theswitching element S4. The gate drive signal of the switching element S1is inverted from the gate drive signal of the switching element S3. Insome embodiments, the gate drive signal of the switching element S1 maybe generated by coupling an inverter between the gate of the switchingelement S1 and the gate of the switching element S3 Likewise, the gatedrive signal of the switching element S2 is inverted from the gate drivesignal of the switching element S4. In some embodiments, the gate drivesignal of the switching element S2 may be generated by coupling aninverter between the gate of the switching element S2 and the gate ofthe switching element S4.

In some embodiments, the duty cycle of the gate drive signal of theswitching element S3 represents the minimum duty cycle of thefull-bridge 106. The minimum duty cycle is about 30% in accordance withsome embodiments. It should be noted the minimum duty cycle shown inFIG. 4 is merely an example. Depending on different applications anddesign needs, the minimum duty cycle may vary accordingly.

It should further be noted the gate drive signals shown in FIG. 4 ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, there may be adead time between two complementary gate drive signals described above.

FIG. 5 illustrates gate drive signals associated with a secondimplementation of the minimum duty cycle control mode in accordance withvarious embodiments of the present disclosure. The minimum duty cyclecontrol mode shown in FIG. 5 is similar to that shown in FIG. 4 exceptthat the gate drive signal of the switching element S3 is notcomplementary to the gate drive signal of the switching element S4. Asshown in FIG. 5, there is a phase shift between the gate drive signal ofthe switching element S3 and the gate drive signal of the switchingelement S4.

In some embodiments, the gate drive signal of the switching element S1is inverted from the gate drive signal of the switching element S3. Thegate drive signal of the switching element S2 is inverted from the gatedrive signal of the switching element S4. In some embodiments, the dutycycle of the gate drive signal of the switching element S3 representsthe minimum duty cycle of the full-bridge. The minimum duty cycle isabout 30% in accordance with some embodiments.

FIG. 6 illustrates gate drive signals associated with the minimumphase-shifted control mode in accordance with various embodiments of thepresent disclosure. In the minimum phase-shifted control mode, there isa phase shift between the gate drive signal of the switching element S3and the gate drive signal of the switching element S4. The gate drivesignal of the switching element S1 is inverted from the gate drivesignal of the switching element S3. The gate drive signal of theswitching element S2 is inverted from the gate drive signal of theswitching element S4.

In some embodiments, the power of the wireless power transfer system 100is delivered from the transmitter coil L1 to the receiver coil L2 duringa first time period from the time instant t1 to the time instant t2, anda second time period from the time instant t3 to the time instant t4.

FIG. 7 illustrates gate drive signals associated with a firstimplementation of the duty cycle control mode in accordance with variousembodiments of the present disclosure. The gate drive signals shown inFIG. 7 are similar to the gate drive signals shown in FIG. 4 except theduty cycle is adjustable so as to control the power delivered from thetransmitter coil L1 to the receiver coil L2. In some embodiments, theduty cycle is in a range from about 30% to about 50%.

FIG. 8 illustrates gate drive signals associated with a secondimplementation of the duty cycle control mode in accordance with variousembodiments of the present disclosure. The gate drive signals shown inFIG. 8 are similar to the gate drive signals shown in FIG. 5 except theduty cycle is adjustable so as to control the power delivered from thetransmitter coil L1 to the receiver coil L2. In some embodiments, theduty cycle is in a range from about 30% to about 50%.

FIG. 9 illustrates gate drive signals associated with the phase-shiftedcontrol mode in accordance with various embodiments of the presentdisclosure. The gate drive signals shown in FIG. 9 are similar to thegate drive signals shown in FIG. 6 except the phase shift between thegate drive signal of the switching element S3 and the gate drive signalof the switching element S4 is adjustable so as to control the powerdelivered from the transmitter coil L1 to the receiver coil L2.

FIG. 10 illustrates gate drive signals having a leading edge alignmentunder the first implementation of the minimum duty cycle control mode inaccordance with various embodiments of the present disclosure. Thehorizontal axis of FIG. 10 represents intervals of time. There may befive vertical axes. The first vertical axis Y1 represents the gate drivesignal of the switching element S1. The second vertical axis Y2represents the gate drive signal of the switching element S2. The thirdvertical axis Y3 represents the gate drive signal of the switchingelement S3. The fourth vertical axis Y4 represents the gate drive signalof the switching element S4. The fifth vertical axis Y5 represents thegate drive signal of the high-side switch Q1 of the power converter 104.

In order to improve the EMI performance of the wireless power transfersystem 100, the leading edge of the gate drive signal of the high-sideswitch Q1 is aligned with the leading edge of the gate drive signal ofthe switching element S3 as shown in FIG. 10. Furthermore, the leadingedge of the high-side switch Q1 is also aligned with the leading edge ofthe gate drive signal of the switching element S4. Such an alignmentbetween the gate drive signal of the power converter 104 and the gatedrive signals of the full-bridge 106 helps to reduce various EMI issuessuch as the beat-frequency issue.

In some embodiments, the switching frequency of the power converter 104is N times greater than that of the full-bridge 106. N is an integer. Insome embodiments, the switching frequency of the full-bridge 106 isabout 120 KHz. The switching frequency of the power converter 104 isabout 1.2 MHz.

FIG. 11 illustrates gate drive signals having a first leading edgealignment under the second implementation of the minimum duty cyclecontrol mode in accordance with various embodiments of the presentdisclosure. The gate drive signal alignment shown in FIG. 11 is similarto that shown in FIG. 10 except that the leading edges of the gate drivesignal of the high-side switch Q1 are aligned with the leading edges ofthe gate drive signals of the switching elements S1-S4, respectively asshown in FIG. 11.

FIG. 12 illustrates gate drive signals having a second leading edgealignment under the second implementation of the minimum duty cyclecontrol mode in accordance with various embodiments of the presentdisclosure. The gate drive signal alignment shown in FIG. 12 is similarto that shown in FIG. 11 except that the leading edges of the gate drivesignal of the high-side switch Q1 are aligned with the leading edges ofthe gate drive signals of the switching elements S3 and S4, respectivelyas shown in FIG. 12.

FIG. 13 illustrates gate drive signals having a first leading edgealignment under the minimum phase-shifted control mode in accordancewith various embodiments of the present disclosure. The gate drivesignal alignment shown in FIG. 13 is similar to that shown in FIG. 10except that the leading edges of the gate drive signal of the high-sideswitch Q1 are aligned with the leading edges of the gate drive signalsof the switching elements S1-S4, respectively as shown in FIG. 13.

FIG. 14 illustrates gate drive signals having a second leading edgealignment under the minimum phase-shifted control mode in accordancewith various embodiments of the present disclosure. The gate drivesignal alignment shown in FIG. 14 is similar to that shown in FIG. 13except that the leading edges of the gate drive signal of the high-sideswitch Q1 are aligned with the leading edges of the gate drive signalsof the switching elements S1 and S3, respectively as shown in FIG. 14.

FIG. 15 illustrates a flow chart of controlling the switches shown inFIG. 2 in accordance with various embodiments of the present disclosure.This flowchart shown in FIG. 15 is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Forexample, various steps illustrated in FIG. 15 may be added, removed,replaced, rearranged and repeated.

Referring back to FIG. 2, the wireless power transfer system 100comprises a power converter 104 and a wireless power transfer device 101connected in cascade between an input power source VIN and a load RL.The power converter 104 is a step-down power converter. The wirelesspower transfer device 101 comprises a full-bridge 106 connected betweenthe output of the power converter 104 and a transmitter coil L1.

Depending on different load conditions, the wireless power transfersystem 100 may operate in different operating modes to improve theperformance of the wireless power transfer system. More particularly, ina light load condition, the power converter 104 is configured toregulate the voltage applied to the full-bridge 106. At the same time,the full-bridge 106 is configured to operate in a minimum duty cyclecontrol mode or a minimum phase-shifted control mode. After the load ofthe wireless power transfer system 100 reaches a predeterminedthreshold, the power converter 104 enters into an always-on mode, and aduty cycle control mode or a phase-shifted control mode may be appliedto the full-bridge 106 to control the power delivered to the load of thewireless power transfer system 100.

At step 1502, the load of the wireless power transfer system 100 isdetected by a suitable sensing apparatus. The detected load is processedby a controller. In particular, the detected load current is comparedwith predetermined current thresholds. In some embodiments, thecontroller may be a digital controller.

At step 1504, the power converter 104 is configured to regulate thevoltage applied to the full-bridge 106 of the wireless power transferdevice when the load current is less than a first current threshold. Insome embodiments, the full-bridge 106 is configured to operate in aminimum phase-shifted control mode when the load current is less thanthe first current threshold. In alternative embodiments, the full-bridge106 is configured to operate in a minimum duty cycle control mode.

It should be noted that the thresholds above are merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. For example, the first current threshold may varydepending on different applications and design needs.

At step 1506, the power converter 104 is configured to operate in analways-on mode when the load current is greater than the first currentthreshold. In some embodiments, the full-bridge 106 is configured tooperate in a phase-shifted control mode when the load current is greaterthan the first current threshold. In alternative embodiments, thefull-bridge 106 is configured to operate in a duty cycle control mode.

At step 1508, the power converter 104 is configured to operate in thealways-on mode when the load current is greater than a second currentthreshold. In some embodiments, the full-bridge 106 is configured tooperate in a maximum phase-shifted control mode when the load current isgreater than the second current threshold. In alternative embodiments,the full-bridge 106 is configured to operate in a maximum duty cyclecontrol mode. The second current threshold is greater than the firstcurrent threshold.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. An apparatus comprising: a power converterconnected between an input power source and a wireless power transfersystem, wherein: the power converter is configured to regulate a voltageapplied to the wireless power transfer system when a load current of thewireless power transfer system is less than a first threshold; and thepower converter is configured to operate in an always-on mode when theload current of the wireless power transfer system is greater than thefirst threshold.
 2. The apparatus of claim 1, wherein: the powerconverter is a step-down converter.
 3. The apparatus of claim 1, whereinthe wireless power transfer system comprises: a full-bridge connectedbetween an output of the power converter and ground; a transmitter coilcoupled to the full-bridge; a receiver coil magnetically coupled to thetransmitter coil; and a rectifier connected to the receiver coil.
 4. Theapparatus of claim 3, wherein: the full-bridge is configured to operatein a minimum duty cycle control mode when the load current of thewireless power transfer system is less than the first threshold; and thefull-bridge is configured to operate in a duty cycle control mode whenthe load current of the wireless power transfer system is greater thanthe first threshold.
 5. The apparatus of claim 3, wherein: thefull-bridge is configured to operate in a minimum phase-shifted controlmode when the load current of the wireless power transfer system is lessthan the first threshold; and the full-bridge is configured to operatein a phase-shifted control mode when the load current of the wirelesspower transfer system is greater than the first threshold.
 6. Theapparatus of claim 3, wherein the rectifier comprises: a full-waverectifier and a capacitor connected in cascade, and wherein thefull-wave rectifier comprises four diodes.
 7. The apparatus of claim 3,further comprising: a resonant capacitor connected between thefull-bridge and the transmitter coil.
 8. The apparatus of claim 3,wherein: the full-bridge is configured to operate in a maximum dutycycle control mode when the load current of the wireless power transfersystem is greater than a second threshold.
 9. The apparatus of claim 8,wherein: the second threshold is greater than the first threshold. 10.The apparatus of claim 1, wherein: the power converter is configured toregulate the voltage applied to the wireless power transfer system suchthat the voltage applied to the wireless power transfer system rises ina linear manner.
 11. A method comprising: detecting a load current of awireless power transfer system comprising a power converter and awireless power transfer device connected in cascade between an inputpower source and a load; configuring the power converter to regulate avoltage applied to the wireless power transfer device when the loadcurrent is less than a first current threshold; and configuring thepower converter to operate in an always-on mode when the load current isgreater than the first current threshold.
 12. The method of claim 11,wherein: the power converter comprises: a high-side switch and alow-side switch connected in series between the input power source andground; and an inductor connected to a common node of the high-sideswitch and the low-side switch; and the wireless power transfer devicecomprises: a full-bridge connected between an output of the powerconverter and ground; a transmitter coil coupled to the full-bridge; aresonant capacitor connected between the full-bridge and the transmittercoil; a receiver coil magnetically coupled to the transmitter coil; anda rectifier connected to the receiver coil.
 13. The method of claim 12,further comprising: configuring the full-bridge to operate in a minimumphase-shifted control mode when the load current is less than the firstcurrent threshold; and configuring the full-bridge to operate in aphase-shifted control mode when the load current is greater than thefirst current threshold.
 14. The method of claim 12, further comprising:configuring the full-bridge to operate in a minimum duty cycle mode whenthe load current is less than the first current threshold; andconfiguring the full-bridge to operate in a duty cycle control mode whenthe load current is greater than the first current threshold.
 15. Themethod of claim 14, wherein: the power converter automatically switchesfrom a voltage regulation mode to the always-on mode after an outputvoltage of the power converter reaches a maximum output voltage of thepower converter; and the power converter automatically switches from thealways-on mode to the voltage regulation mode after the full-bridgereaches a clamped duty cycle of the full-bridge.
 16. The method of claim14, wherein: a switching frequency of the power converter is N timesgreater than a switching frequency of the full-bridge, and wherein N isan integer.
 17. A system comprising: a converter comprising: a high-sideswitch and a low-side switch connected in series between an input powersource and ground; an inductor connected to a common node of thehigh-side switch and the low-side switch; and an output capacitorconnected to the inductor; and a wireless power transfer devicecomprising: a full-bridge connected between an output of the converterand ground; a transmitter coil coupled to the full-bridge; a resonantcapacitor connected between the full-bridge and the transmitter coil; areceiver coil magnetically coupled to the transmitter coil; and arectifier connected to the receiver coil, wherein the converter isconfigured to operate in two different operation modes in response todifferent load currents of the wireless power transfer device.
 18. Thesystem of claim 17, wherein: the converter is configured to regulate avoltage applied to the full-bridge when a load current of the wirelesspower transfer device is less than a first threshold; and the converteris configured to operate in a bypass mode when the load current of thewireless power transfer device is greater than the first threshold, andwherein during the bypass mode, the voltage applied to the full-bridgeis substantially equal to an output voltage of the input power source.19. The system of claim 18, wherein: the full-bridge is configured tooperate in a minimum duty cycle mode when the load current of thewireless power transfer device is less than the first threshold; and thefull-bridge is configured to operate in a duty cycle control mode whenthe load current of the wireless power transfer device is greater thanthe first threshold.
 20. The system of claim 18, wherein: thefull-bridge is configured to operate in a minimum phase-shifted controlmode when the load current of the wireless power transfer device is lessthan the first threshold; and the full-bridge is configured to operatein a phase-shifted control mode when the load current of the wirelesspower transfer device is greater than the first threshold.